1. Field of the Invention
The present invention, in general relates to electrical contacts and, more particularly, to electrical contacts that provide minimal insertion-removal force and which are tolerant of axial misalignment of the contacts.
The pin and socket configuration of electrical contacts is common in a variety of industries and frequently includes a solid pin contact that mates to a slotted or “split tine” type of a socket contact.
The split tine socket is typically deformed radially inward after machining in order to achieve an adequate normal force when mated to the pin contact. This deformation is sometimes referred to as a “set”.
Another method of achieving inward normal force on the pin contact is the use of a separate coil spring or “C” clip spring that is wrapped around the tip of the tines and which tends to urge them inward, toward the pin.
A common type of problem that occurs generally with these types of contacts involves achieving a consistent normal force. A consistent normal force results in consistent mate/unmate forces and also in a consistent, and preferably, low voltage drop across the connection.
Maintaining a consistent normal force over the life of the contact, which may be subjected to harsh environments and abuse, remains a vexing problem in the industry.
These issues are especially important when high currents are involved, such as when fast charging electric vehicles. Also, the connectors that utilize these types of contacts may be handled by personnel with limited strength, as is discussed in greater detail hereinafter.
It is desirable that such contacts have low mate and unmate forces and that those forces remain relatively constant throughout the connector's useful service life.
If the mate/unmate force is too high, the connector, having a plurality of contacts, may be unusable by some people of limited strength. This is especially true for connectors that carry significant amounts of current.
Conversely, if the contacts loosen excessively over time (i.e., if the normal force decreases substantially), a resulting increase in resistance and therefore voltage drop can occur. This, in turn, will cause a rise in temperature and may result in an unsafe situation.
Setting the tines of the contact requires using a material with a sufficiently low yield strength such that sufficient permanent deformation can take place within the constraints of the slit width, in order to achieve the desired normal force.
Unfortunately, permanent deformation in the outward radial direction or “loosening” can occur over the life of the contact resulting in a reduction of normal force and an increase in voltage drop. Loosening is a common problem with connectors that are mated and unmated repeatedly.
This is especially true when the connector design allows a rocking motion to be used as an aid in mating and unmating. Installing an external helper spring wrapped around the tip of the tines can help alleviate this problem because the spring is made of a high yield strength material that is resistant to permanent deformation.
However, the external spring's spring rate, dimensions, and frictional characteristics contribute to a variation in normal force. Also, the frictional characteristics of the spring/tine interface are subject to change over the life of the contact, especially in harsh environments.
Accordingly, the selection of an external helper spring must take all of these variables into account and, therefore, one must be selected that will still provide the minimal normal force that is required over the life of the contact. Selecting a helper spring that can supply the necessary normal force after having taken into account the variables that are involved tends to increase the mate/unmate forces that are required. It also adds one more component part (i.e., the external spring) to the assembly of each contact.
Also, prior art design of contacts, especially high current contacts, has been based on the prevailing assumption that it is desirable to maximize the contact area intermediate a pin and a socket. The more contact area that occurs at the interface between the pin and the socket, it has been believed, will increase the opportunity for current to flow. It has been thought that current flow will occur at least somewhere wherever there is the potential for physical contact to occur, so the greater the potential for that contact to occur and to occur in as many places as possible, became the essence of optimum high current contact design theory.
It was further believed that a great amount of surface area for contact is absolutely necessary to support high current loading through the connector. The problem with maximizing contact area is that, for any given tine to pin pressure (i.e., normal force), a greater area for contact results in less normal force being applied at any given location. This tends to result in random spots of contact occurring. If contact is random, there is little assurance that any mechanical “wiping” will clean the pin and tines at the exact areas where physical contact will occur.
This, it has been found, decreases the current carrying ability of a contact over its life because oxidation that occurs is not optimally cleaned by the wiping action of the tine with the pin. Also a lower normal force also tends to increase electrical resistance in general.
With this to serve as a general background, there is one further condition that exacerbates the problem of high mate and unmate forces, and that is axial misalignment of the contacts that occurs after molding.
It is common that such types of connectors have rubber molded around the outside of the contacts. The contact position at the face of the rubber connector is held to tight tolerances by the tooling. This is intended to keep the longitudinal axes of the contacts as contained within the two mating connectors (i.e., the male and female halves) in parallel alignment with respect to each other prior to the molding process.
However, the molding process subjects the contacts to lateral loading forces which affect the individual contacts to varying degrees. A “fluid rubber” is injected into the molds and this exerts a force that bears upon the individual contacts. Misalignment can also occur in response to tensile forces on the wire that is crimped into the contact due to the molding pressure bearing on the wire where it exits from the mold.
Also, when the rubber cures it shrinks slightly and this tends to pull the contacts together toward the back portion of each contact.
Regardless of the causes, the axial parallelism of the individual contacts typically may vary from one to three degrees from normal (i.e., parallel).
This substantially tends to increase the mate/unmate forces. If it were not for axial misalignment, a six contact connector in which each of the contacts requires five pounds of force to mate or unmate would require six times five, or thirty pounds of force.
Because of axial misalignment, the actual force may be double that, or sixty pounds. This is a substantial problem, that heretofore has gone unresolved, especially for high current applications involving repeated mate and unmate connections per day.
As mentioned hereinabove, axial misalignment affects the various contacts in varying degrees. Those contacts that are disposed at the perimeter of the connector tend to experience greater misalignment than those that are disposed toward the center of the connector.
Also, all contacts are not the same size. The length of the pin and sockets are carefully chosen in these types of connectors so that an interlock feature will function properly. To accomplish this, the power contacts are typically longer than “control contacts”. The control contacts are used to enable or disable current flow through the power contacts by closing or opening a circuit which activates a contactor or a relay. The control contacts are the last ones to physically connect during mating and the first to disconnect during unmating. This helps prevent arcing that would otherwise occur at the power contacts if the high-current power contacts were themselves used to make or break the current flow.
As a result of different locations as well as different contact sizes, there will be a variance in the force required to mate and unmate each connector. If six contacts are used that each average ten pounds of force for each contact (without axial misalignment), perhaps two of them (near the center) achieve ten pounds of force each, two others require fifteen pounds of force, and the remaining two require twenty pounds of force. Instead of the target mate/unmate force of sixty pounds, the actual force required may be ninety pounds, or one and one-half times what is desired.
As was mentioned briefly hereinabove, whenever the mate or unmate force is high, people tend to rock the plug back and forth as it is inserted into the socket. This tends to urge the tines outward toward a hood that surrounds the tines. The act of further opening the tines by itself tends to increase the force that is required. A lack of any clearance intermediate the interior of the tine and the exterior of the pin applies a force (during rocking or if axial misalignment of the contacts is present) that urges the tines outward. The act of further opening the tines (by the nose of the pin) tends to contribute toward premature wear of both the tine and the pin at the point of contact.
A lack of clearance intermediate the interior of the hood and the exterior of the tines not only makes for difficult rocking (because there is not sufficient space for the tines to be urged further away from a center axis of the contact when the hood is contacted by the tines) but it is likely to damage the contacts (i.e., most likely the tines).
This is because attempting to rock the male connector half (with the pins) as it is being inserted into the female connector half (with the sockets) when the tines are in contact with the hood subjects the tines to great stresses (at the point of contact by the pins) and this force can exceed the elastic limit of the tines, thereby deforming them and possibly changing their “set”. This degrades the normal forces that are applied by the tines to the pin, possibly reducing the current-carrying capacity of the connector as well as its useful life.
Accordingly, there exists today a need for a low force electrical contact that is durable and reliable, adaptable for use in harsh environments, requires a minimal mate/unmate force, is tolerant of axial misalignment and is capable of carrying high currents.
2. Description of Prior Art
Electrical contacts are, in general, well known. While the structural arrangements of the known types of devices, at first appearance, may have similarities with the present invention, they differ in material respects. These differences, which will be described in more detail hereinafter, are essential for the effective use of the invention and which admit of the advantages that are not available with the prior devices.